Fumaroles are known to generate seismic and infrasonic tremor, but this fumarolic tremor has so far received little attention. Seismic records taken near the Pisciarelli fumarole, a vigorously degassing vent of the restless Campi Flegrei volcano in Italy, reveal a fumarole-sourced tremor whose amplitude has recently intensified. We use independent geochemical evidence to interpret this fumarolic tremor for the first time quantitatively. We find that the temporal increase in fumarolic tremor RSAM (real-time seismic-amplitude measurement) quantitatively correlates with increases in independent proxies of fumarole activity, including the CO2 concentrations in the fumarole’s atmospheric plume, the fumarole composition (CO/CO2 and H2/H2O ratios), and temperature (T) and pressure (P) conditions of the source hydrothermal system. This association between RSAM and geochemical data suggests that the current escalation in fumarolic tremor is driven by increased gas transport and venting from a hydrothermal system that evolves toward higher T-P conditions. More widely, our results suggest that monitoring the tremor generated by fumaroles can pave the way to real-time tracking and interpreting the evolution of an ongoing hydrothermal-magmatic unrest, thus complementing information from periodic gas surveys. Our novel method of fumarole monitoring, based on well-established seismic techniques, is potentially suitable for capturing signals of unrest at any active volcano, including those in remote regions.